The changes in electrical impedance were studied in cats subjected to left MCA occlusion, either permanent or followed by the release of occlusion. The main findings indicate that shortly following MCA occlusion there is a progressive increase in impedance, reflecting the onset of cytotoxic edema in the ischemic regions. The breakdown of the BBB is reflected in decrease of impedance. The correlation of impedance measurements with those of rCBF in the same experiment should greatly elucidate the dynamics of ischemic brain edema.